With continued improvement and maturing in technology, various plastic optical fibers have been manufactured and/or produced. These plastic optical fibers (POFs) have been making their headways into system application in a variety of fields such as, for example, home theaters, automobiles, and aerospace industry. In the meantime, there has been an increasing demand for optical fiber couplers, in particular plastic optical fiber couplers (or plastic fiber couplers in short) that are considered to be one of the several key components or devices that will enable essential configurations of systems that utilize plastic optical fibers.
The manufacturing of plastic fiber couplers (PFCs) started about ten (10) years ago, but so far little practical progress has been made. This is because, at least partially, that fused biconical taper technology, which was developed specifically for silica fibers, is not readily suitable for and therefore can not be directly applied to plastic optical fibers in making plastic fiber couplers. Even though there may have been scientific reports, from universities and research institutes alike, on some plastic fiber couplers, most of these reports focus mainly on couplers that have low number of input and/or output ports. Without any suitable replacement for the fused biconical taper technology, it has been very difficult to manufacture plastic fiber couplers, in particular those with high number of input and/or output ports.
For example, among some of the reported plastic fiber couplers there is a seven (7) port reflective type plastic fiber coupler as being demonstratively illustrated in FIG. 1(a)-FIG. 1(c). Hereinafter, FIG. 1(a)-FIG. 1(c) may be collectively referred to as FIG. 1 and similar ways of referencing may be used for other figures as well. This seven (7) port reflective type plastic fiber coupler is based upon a mixing rod technology. More specifically, FIG. 1(a) demonstratively illustrates an overlapping view of cross-sections of a fiber bundle 101 and a mixing rod 102 of a plastic fiber coupler 100. Fiber bundle 101 is composed of seven plastic optical fibers (POF-11, POF-12, etc.), used as input and/or output ports of plastic fiber coupler 100. The seven POFs may be arranged or stacked together in a way that one POF (e.g., POF-11) is positioned or situated in the center of fiber bundle 101 and surrounded by six (6) other POFs, including POF-12, of same or similar size.
FIG. 1(b) is a demonstrative perspective view of fiber bundle 101 shown in FIG. 1(a). The seven POFs of fiber bundle 101 may have a common cross-sectional area S1. During the process of manufacturing plastic fiber coupler 100, cross-sectional area S1 may be prepared to have a flat and smooth surface and then be attached or glued to a cross-section of mixing rod 102 as being further described below.
FIG. 1(c) is a demonstrative perspective view of mixing rod 102 used in making plastic fiber coupler 100. Mixing rod 102 may be another piece of POF having a diameter around 3 mm; a length ranging from around 3 cm to around 20 cm; a cladding layer S3; and first and second cross-sectional areas S2 and S4. In making reflective plastic fiber coupler 100, cross-sectional area S2 of mixing rod 102 may be prepared to have a flat and smooth surface, and then be glued or attached to cross-sectional area S1 of fiber bundle 101. At the other end of mixing rod 102, cross-sectional area S4 may be applied with a reflective coating or film to act like a mirror to light incident thereupon. Thereby, for example, when there is a light from any one of the seven POFs (e.g., POF-11, or POF-12) being launched into mixing rod 102, the light may be reflected back into all of the seven POFs, effectively creating optical coupling effect among the seven POFs.
However, the seven-port reflective type plastic fiber coupler 100 normally has a large insertion loss. Even though optical light may be fully, close to 100%, coupled from one of the seven POFs into mixing rod 102, when the light is reflected back by cross-sectional area S4 of mixing rod 102 and coupled into the seven POFs, because of mismatch in cross-sectional areas, in particular with cross-sectional area S2 of mixing rod 102 being larger than cross-sectional area S1 which is a sum of the total seven POFs, a portion of the light or optical energy may inevitably get launched into areas/spaces outside the seven POFs and lost.
In addition to large insertion loss, optical light may not get uniformly coupled back into the seven POFs. This is because, for example as shown in FIG. 1(a), POF-11 and POF-12 may be at different locations of cross-sectional area S1, relative to the center thereof. Since optical light concentrates more around the center area of mixing rod 102 than around the edge areas, more light are likely to be coupled back into POF-11 in the center than into POF-12 at the edge of fiber bundle 101. It is generally understood that with the increase of number of ports, such insertion loss and uniformity issue as being discussed above may further degrade or become worse.
FIG. 2(a)-2(c) are demonstrative illustrations of another reflective type plastic fiber coupler 200 made of a bundle of nineteen (19) plastic optical fibers and a mixing rod as is known in the art. In particular, when the nineteen POFs of fiber bundle 201 are stacked in a 3-4-5-4-3 layered fashion as shown in FIG. 2(a), plastic optical fibers such as POF-21, POF-22, POF-23, and POF-24 may be situated or positioned at different locations across cross-sectional area S1 of fiber bundle 201 (thus cross-sectional area S2 of mixing rod 202 which is glued to S1) relative to the center thereof. Uniformity of optical light, that is reflected back from cross-sectional area S4 of mixing rod 202, guided by a cladding S3, and coupled into the nineteen input/output POFs, is expected to become worse than that in the seven-port plastic fiber coupler 100. It is known that increasing the length of mixing rod 202 will generally not help improving the uniformity of optical light being coupled into the input/output POFs.